Method for reducing the resistivity of the corrosion-induced oxide layer, and applications

ABSTRACT

A method for reducing the electrical resistance caused by a corrosion oxide coating at the current-carrying transition of a chromium steel component includes annealing the chromium steel component a temperature of at least 950° C. and removing any oxide coating that develops during said annealing before the component carries current.

This application is the U.S. national stage application ofPCT/DE98/02538, and claims priority of German patent document 197 38405.4, filed on Sep. 3, 1997.

BACKGROUND AND SUMMARY OF INVENTION

The invention relates to a method for reducing the electrical resistanceon the current-carrying transition to a component made of chromium steeldue to a corrosion-caused surface layer of oxide on the component. Italso has applications of this method as its subject.

In many technical apparatus chromium steel is used for many kinds ofcomponents for reasons of strength and corrosion resistance. Thismaterial resists corrosion attack by forming a protective chromium oxidecoating. The chromium oxide coating is a good barrier against diffusionand thus prevents corrosion attack. Especially in high-temperatureapplications the chromium oxide coating leads to a great reduction ofoxide growth and thus to a lasting protection against the destruction ofthe material.

If chromium steel, however, is used for electric current carryingcomponents in high-temperature applications in an oxidizing atmosphere,the necessary formation of the chromium oxide coating causes anelectrical resistance caused by this coating, which in turn leads tovoltage losses and thus to a lowering of the efficiency of the apparatusin question.

An example of a component which is exposed to an oxidizing atmosphere athigh temperatures is the cathode current collector of a molten carbonatefuel cell (MCFC).

Molten carbonate fuel cells consist essentially of a porous cathode anda porous anode and a matrix which is imbibed with a molten electrolyte,namely a eutectic mixture containing a lithium carbonate and otheralkali carbonates which is in contact with the electrodes. To carry theelectrochemically produced current, current collectors are in contactwith the cathode and the anode, and they are usually corrugated in orderalso to form a gas transport space, and namely for carrying air or otheroxygen-containing gas at the cathode and a fuel gas at the anode. Thefuel cell is operated at a temperature of 500 to 800° C. The cathodecurrent collector is exposed to severe corrosive influences by contactwith the oxygen-containing gas and the molten carbonate as well as bythe high temperature. In spite of the use of chromium steel, an oxidecoating thus forms on the surface of the cathode current collector,which leads to a high resistance to transition between the cathodecurrent collector and the cathode and thus to high power losses in themolten carbonate fuel cell.

To lower this transition resistance, it is proposed according to DE 19532 791 A1 to apply a noble metal, such as gold or platinum, to thecathode current collector at the points of contact with the cathode.Aside from the high costs of the noble metals, this method has thedisadvantage that a diffusion barrier must still be placed between thethin noble metal coating and the chromium steel cathode currentcollector in order to prevent the diffusion of the noble metal into thechromium steel. Thus several coating steps are necessary.

The invention is addressed to the problem of offering a method wherebythe electrical resistance at the transition to a current-carryingchromium steel component due to a surface oxide coating caused bycorrosion on the component can be reduced in a simple manner.

This is achieved according to the invention by annealing the chromiumsteel component a temperature of at least 950° C., and removing anyoxide coating that develops during said annealing before the componentcarries current.

DETAILED DESCRIPTION OF THE INVENTION

By the method of the invention the chromium steel component in questionis annealed. The annealing must be performed either with the absoluteexclusion of oxygen, or any oxide coating formed by the annealing mustbe removed entirely at least at the current-carrying transition points,before the component is installed.

A surface oxide coating is necessarily again formed on thecurrent-carrying chromium steel component treated according to theinvention when it is used in an oxidizing temperature, especially athigh temperatures. Amazingly, however, this oxide coating has asubstantially reduced electrical resistance, and indeed the transitionresistance compared with a chromium steel component of the samecomposition, but one that has not been annealed by the method of theinvention, is reduced approximately ten-fold.

It is furthermore amazing that, despite the lowering of the electricaltransition resistance the oxide coating formed on the component whenused in an oxidizing atmosphere resists the attack of corrosion in thesame manner as a chromium steel component of the same composition, butone which has not been annealed in an oxygen-free atmosphere accordingto the invention.

The phenomenon that occurs in the practice of the invention, namely thatthe surface oxide coating formed when the component is used in anoxidizing atmosphere has on the one hand a high corrosion resistance,but on the other hand a low electrical resistance, cannot be explained.It is true that the grain structure of the chromium steel changes in theannealing, so that it may be thought that less chromium diffuses out ofthe steel to the surface and thus the formation of chromium oxide at thesurface is repressed.

On the other hand, however, it is precisely an oxide coating containingchromium oxide on the surface is considered as a requirement for theanticorrosive properties of a chromium steel.

In other words, it has been found that, in comparison to oxide coatingsforming on untreated chromium steel the electrical resistance issurprisingly lowered by the method of the invention by about one orderof magnitude. Detailed studies by physical methods show a decideddifference in the composition and structure of the oxide coatings formedand those of untreated chromium steel of the same composition. Thechromium content in the case of the oxide coating that has grown on thesteel treated according to the invention is decidedly lower through theentire coating than the oxide coating grown on the same but untreatedsteel. The chromium content of the untreated steel and the steelaccording to the invention itself is equal in volume, so that the causeof the combination of good corrosion resistance and low electricalresistant is not simply a lowering of the chromium content. For issimply the chromium content of the steel is lowered, a lowering of theelectrical resistance is indeed obtained, but at the same time adefinite deterioration of the corrosion resistance is found. If thetreated chromium steel is a steel of the material number 1.4404 (AISI316 L), the steel treated by the method of the invention shows incomparison with untreated steel a coarse structure and a lowering of thechromium and magnesium content at the surface.

Preferably, a chromium steel with a chromium content of 10 to 22 wt.-%,especially 15 to 19 wt.-%, and with very special preference less than 17wt.-%.

The other components of the alloy steel can be in weight-percent:

0 to 2% carbon, silicon, phosphorus and/or sulfur

0 to 20% manganese

0 to 10% molybdenum

0 to 20% nickel

0 to 15% cobalt

less than 0.5% aluminum, yttrium, titanium and/or cerium.

In particular the content of aluminum, yttrium, titanium and/or ceriumshould be less than 0.05%, for these elements form oxides with a veryhigh electrical resistance on the surface of the component.

Especially steel of the material number 1.4404 has proven suitable forthe method of the invention. This steel has the following composition inweight-percent:

C≦0.030

Si≦1.00

Mn≦2.00

P≦0.045

S≦0.030

Cr 16.5-18.5

Mo 2.00-2.50

Ni 11.0-14.0

Remainder iron and impurities caused in production, or

C≦9,93

Si≦1.50

Mn≦1.50

P≦0.035

S≦0.020

Cr 17.0-20.0

Mo 2.00-3.00

Ni 9.00-13.0

Remainder iron and impurities caused in production.

The annealing of the chromium steel component is performed in the methodof the invention at a temperature of at least 950° C., preferably in atemperature range between 1050 and 1400° C., annealing for at least onehour.

To prevent a thick oxide coating from forming during the annealingprocess, the annealing process must be performed in vacuo or in anoxygen-free atmosphere, for example in pure hydrogen. It is notsufficient to anneal under a common shielding gas, i.e., noble gas,nitrogen or forming gas, because at the annealing temperature the oxygenpresent, even though in extremely small amounts, leads in any case tothe formation of an oxide coating which does not have the desiredproperties.

Also it is possible to anneal in other atmospheres, or even air, ifafter this annealing treatment the oxide coating is carefully removedagain, at least from the current-carrying transition points on thecomponent.

If the chromium steel components thus treated are used in a temperaturerange of 500 to 800° C. in an oxidizing atmosphere, an oxide coatingforms which is composed of several phases. In this case it is a mixtureof many different iron oxides, nickel chromium oxides, and spinels.

Since, as mentioned above, especially the cathode current collectors ofmolten carbonate fuel cells are exposed to severe corrosive attack, buton the other hand must have a low electrical transition resistance atthe points of contact with the cathode, the method of the invention isappropriate, for example, for the production of cathode currentcollectors of chromium steel for molten carbonate fuel cells.

What is claimed is:
 1. A method for making a molten carbonate fuel cell,comprising: annealing a chromium steel component at a temperature of atleast 950° C.; removing any oxide coating that develops during saidannealing, thereby reducing electrical resistance caused by a corrosionoxide coating at a current-carrying transition of the chromium steelcomponent; and installing the chromium steel component as a cathodecurrent collector in a molten carbonate fuel cell.
 2. A method accordingto claim 1, wherein the chromium steel component has a chromium contentof 10 to 22 wt.-%.
 3. A method according to claim 2, wherein thechromium content is 15 to 19 wt.-%.
 4. A method according to claim 1,wherein the chromium steel component contains, in weight percent: 0 to2% of at least one of carbon, silicon, phosphorus or 0 to 20% manganese;0 to 20% nickel; 0 to 15% cobalt; and less than 0.5% of at least one ofaluminum, yttrium, titanium or cerium.
 5. A method according to claim 1,wherein the chromium steel component has material number AISI 316 L. 6.A method according to claim 1, wherein said annealing is at atemperature between 1050 and 1400° C.
 7. A method according to claim 1,said annealing is performed for at least one hour.
 8. A method accordingto claim 1, wherein said annealing is performed in vacuo or in anoxygen-free atmosphere.
 9. A method according to claim 8, wherein theoxygen-free atmosphere is hydrogen.